Device for correcting a receiving signal

ABSTRACT

The invention relates to a device for correcting a receiver signal which is associated with an emission signal transmitted in a distorted transmission system. The emission signal comprises periods which can be determined by analyzing the received signal wherein determined properties are exhibited which are suitable for adjusting the correction. According to one embodiment, the device comprises a component for adjusting the correction based upon an analysis of the received signal and a component for monitoring and enabling the adjusting component when the received signal associated with the transmission signal exhibits certain characteristics.

[0001] The present invention relates to a device for equalizing areceived signal and in particular to a device for equalizing a receivedsignal of a transmission system.

[0002] In the case of digital transmission methods, adaptiveequalization is an effective measure to compensate in particular fortime-dependent distortions on the transmission link, such as due toageing, temperature fluctuations or changing connection configurations.In view of the complexities available in the case of modern CMOStechnologies for digital signal processing, such as for example thenumber of available logical gates and the associated signal processingpossibilities, it is also attractive for cost reasons to reduce therequirements with regard to the tolerances of analog components/modulesand to compensate for the resulting system imperfections that are causedby process fluctuations, but also by time-variant external influences,such as for example temperature fluctuations, by means of an adaptiveequalizer. Time-variant phenomena, such as for instance the changing ofthe transmission function of analog components on account of atemperature drift, in this case occur on a time scale which is severalorders of magnitude longer than the symbol period. This is therefore aquasi-static problem, i.e. the optimum equalization, such as for examplethe optimum coefficient setting for a filter, of an equalizer isvirtually time-independent. Only the spectral properties of thetransmission spectrum are time-variant under some circumstances.

[0003] The setting of an adaptive equalizer can be carried out with theaid of the transmission of a known data sequence (data-aided), such asfor example a preamble. In the case of many transmission methods,however, in particular in the case of continuous transmission bycontrast with burst mode, or for example block-based methods, such asmulti-carrier transmission, the transmission of such a known datasequence is not envisaged. An adaptive equalizer may thereforealternatively be constructed in such a way that the information for thesetting of the adaptive equalizer is derived from the received andpossibly falsified data alone. This presupposes that the transmitteddata sequence has particular properties.

[0004] One particular property of such a data sequence is, for example,that the input spectrum must cover the entire frequency range of theequalizer. This property is not ensured for example in the case ofdecimating equalizers, which reduce the data rate, for example bysubsampling of the signal, from the input to the output. The frequencyrange of the equalizer in this case extends to f_(sampling)/2, wheref_(sampling) is for example an integral multiple of f_(symbol). Thesignal energy is essentially restricted to a range <f_(symbol)/2. Thelacking signal energy above f_(symbol)/2 may cause a problem which isknown by the term “tap wandering” or coefficient wandering. f_(symbol)here is the symbol frequency of transmitted symbols, which arerepresented for example by voltage levels in a transmitted signal.

[0005] A further particular property of the above-mentioned datasequence is, for example, that the successive data must not becorrelated, i.e. the input spectrum must be “white” and must ideallyhave a flat frequency profile with a low-pass characteristic. Thisproperty can be ensured if a pseudo-random sequence is generated fromthe data before the transmission by means of a scrambler.

[0006] There are, however, a whole series of applications in whichnothing can be stated about the properties of the transmission spectrumover time. The reason for this is that the coding schemes used arechosen merely with a view to ensuring special properties of the system,such as for example facilitating clock recovery or the absence of adirect component in the transmission spectrum. In particular, ascrambler is not usually provided.

[0007] One example of such an application is digital data transmissionwithin the PDH (=Plesiochronous Digital Hierarchy) according to theT_(x) standard or DS_(x) standard in the USA (x=1:1.544 Mbit/s datatransmission rate; x=3:44.736 Mbit/s data transmission rate) or the Exstandard in Europe (x=1:2.048 Mbit/s data transmission rate; x=3:34.368Mbit/s data transmission rate), for which components are currentlyrequired in large volume (large numbers of items). A further example ofsuch an application is the digital transmission between a physical layer(PHY; PHY=Physical Layer) of a 10 gigabit/s transmission via glass fibreand the associated intermediate access layers (MAC; MAC=Medium AccessLayer) via the so-called “Ten Gigabit Attachment Unit Interface (XAUI)”,which are specified within the IEEE Standard P802.3ae. The aim is topermit the greatest possible physical separation of these layers. Thisapplication will be of enormous significance in the near future.

[0008] Furthermore, in the case of many systems, the transmissionspectrum deviates more or less significantly from the ideal of a “flat”baseband spectrum with a low-pass character. When a line coding as inthe aforementioned Tx/Ex transmission is used, in the case of which thetransmission spectrum is not intended to have any direct component,there occurs for instance a passband characteristic, in the case ofwhich the spectrum in the pass band is also not flat.

[0009] Individual effects, such as the frequency-dependent attenuationin the case of transmission via two-wire copper lines, which isdetermined by the length of the line, or the distortions which areinduced by tolerances in the cut-off frequency of an analoganti-aliasing filter with a known transmission function, can becompensated by means of an equalizer which is fixed but can be set, inthe form of a programmable filter.

[0010] The setting of such an equalizer on a chip can take place on theone hand externally, such as for example by the selection ofcoefficients from sets of coefficients which are stored on the samechip, or by external programming. In this case, for example, a settingat which the lowest bit error rate is achieved may be chosen. However,also possible on the other hand is a setting by a control unit on thechip, which can select between the coefficients which are stored on thechip by means of a suitable criterion.

[0011] In both cases, the stored coefficients can approximate thetransmission function that is inverse to the distortion and bedetermined for instance by minimizing a suitable cost function, such asfor example a criterion of a least mean square error (LMS; LMS=LeastMean Square). In the case of many applications, however, there arepractical limits to such a procedure, since only a limited number ofsets of coefficients can be stored. Such a procedure may be usefulwhenever it is intended to compensate essentially for a dominantdistortion, which moreover is determined by few parameters. Time-variantphenomena, such as for example temperature fluctuations, ageing etc.,cannot generally be taken into account in this way.

[0012] A problem in the prior art is therefore that the known settableequalizers can only be used to a limited extent to compensate fordistortions of, for example, a transmission link and analog components.

[0013] A further problem in the prior art is that virtually notime-variant distortions, caused for example by temperaturefluctuations, ageing etc., can be compensated with the known settableequalizers.

[0014] The object of the present invention is consequently to provide atransmission system which comprises a device for equalizing the receivedsignal permitting flexible equalization of time-variant distortions thatare caused in particular by the transmission system. In this case, thedevice for equalizing a received signal must be designed in such a waythat equalization is also achieved when nothing can be stated a prioriabout the time with respect to the spectrum of the received signal.

[0015] This object is achieved by a device for equalizing a receivedsignal according to claim 1, a transmission system according to claim 28and a method according to claim 29.

[0016] The idea on which the present invention is based is thatautomatic adaptation methods which have proved successful in the case ofcontinuous transmission of data with a white spectrum are combined witha time-limited, that is block-by-block, evaluation of the data stream,in a way similar to in the case of data-aided methods, in which thetransmitted data are known.

[0017] By contrast with such methods, which for example use a knownframe structure and a preamble transmitted along with it, in the case ofthe proposed method the blocks of the received signal that can be usedfor the adaptation are determined automatically. Such a method with acorresponding extra effort for the evaluation of the received signal andthe corresponding activation of the adaptation is attractive in view ofthe technological advancement (or the increasing available complexity ona chip).

[0018] One advantage of the present invention is that, with the aid ofthe present invention, the advantages presented above of adaptiveequalization can also be used in the case of transmission methods inwhich nothing can be stated about the properties of the transmissionspectrum over time. The invention can also be used in those cases inwhich there is no transmitted signal at all in a temporallyunforeseeable way.

[0019] A further advantage of the present invention is that it can beextended in a simple manner to be used in cases in which the convergencebehavior of the adaptive equalization is poor.

[0020] Advantageous developments and improvements of the devicespecified in claim 1 can be found in the subclaims.

[0021] According to a preferred development of the invention, the samealso has a first means for taking decisions and for supplying a firstestimate signal, which estimates the transmitted symbols of thetransmitted signal and supplies the estimated symbols with the firstestimate signal.

[0022] According to a further preferred development of the device, thesame also has a first means for supplying a first error signal, whichdetermines the first error signal from the difference between the firstestimate signal, which is supplied by the first means for takingdecisions, and the first equalized received signal, which is supplied bythe first means for equalizing.

[0023] According to a further preferred development of the invention,the means for setting the settable equalization determines the firstsettable equalization of the first means for equalizing the receivedsignal from the received signal and the first error signal.

[0024] According to a further preferred development of the device, themeans for setting the settable equalization derives the first settableequalization of the first means for equalizing the received signal fromthe correlation of the received signal and the first error signal.

[0025] According to a further preferred development of the device, themeans for monitoring the received signal switches the first error signalthrough to the means for setting the settable equalization and activatesthe means for setting the settable equalization when the transmittedsignal assigned to the received signal has the particular properties.

[0026] According to a further preferred development of the device, thesame also has a second means for equalizing the received signal and forsupplying a second equalized received signal, the second means forequalizing the received signal having a second settable equalization.

[0027] According to a further preferred development of the device, thesame also has a second means for taking decisions and for supplying asecond estimate signal, which estimates the transmitted symbols of thetransmitted signal and supplies the estimated symbols in the secondestimate signal.

[0028] According to a further preferred development of the device, thesame also has a second means for supplying a second error signal, whichdetermines the second error signal from the difference between thesecond estimate signal, which is supplied by the second means for takingdecisions, and the second equalized received signal, which is suppliedby the second means for equalizing the received signal.

[0029] According to a further preferred development of the device, themeans for setting the settable equalization determines the secondsettable equalization of the second means for equalizing the receivedsignal from the received signal and the second error signal.

[0030] According to a further preferred development of the device, themeans for setting the settable equalization derives the second settableequalization of the second means for equalizing the received signal fromthe correlation of the received signal and the second error signal.

[0031] According to a further preferred development of the device, themeans for monitoring the received signal switches the second errorsignal through to the means for setting the settable equalization andactivates the means for setting the settable equalization when thetransmitted signal assigned to the received signal has the particularproperties.

[0032] According to a further preferred development of the device, thesame also has a means for comparing, which compares a first qualitylevel, which is derived from the first error signal, and a secondquality level, which is derived from the second error signal, and, ifthe second quality level is greater than the first quality level,instigates that the set equalization of the second means for equalizingthe received signal is supplied by the means for setting the settableequalization to the first means for equalizing the received signal.

[0033] According to a further preferred development of the device,upstream of the means for comparing there are arranged a first and asecond means for ascertaining a quality level, which derive the firstand second quality levels from the first error signal and the seconderror signal and supply them to the means for comparing.

[0034] According to a further preferred development of the device, thefirst and second quality levels are derived from the mean square errorof the first error signal and the second error signal.

[0035] According to a further preferred development of the device, themeans for monitoring the received signal monitors optionally either thefirst equalized received signal, which is supplied by the first meansfor equalizing the received signal, or the first estimate signal, whichis supplied by the first means for taking decisions.

[0036] According to a further preferred development of the device, themeans for monitoring the received signal monitors optionally either thesecond equalized received signal, which is supplied by the second meansfor equalizing the received signal, or the second estimate signal, whichis supplied by the second means for taking decisions.

[0037] According to a further preferred development of the device, thesame also has a first shaping filter, which supplies a shaped receivedsignal to the second means for equalizing the received signal and themeans for setting the settable equalization, and a second shapingfilter, which has a filter function corresponding to the first shapingfilter and supplies a shaped second estimate signal to the means forsupplying a second error signal.

[0038] According to a further preferred development of the device, thesame also has a first shaping filter, which supplies a shaped receivedsignal to the means for setting the settable equalization, and a secondshaping filter, which has a filter function identical to the firstshaping filter and supplies a shaped second error signal to the meansfor monitoring the received signal.

[0039] According to a further preferred development of the device, thefirst and/or second means for equalizing the received signal have aprogrammable filter, the equalization of which can be set by means offilter coefficients by the means for setting the settable equalization.

[0040] According to a further preferred development of the device, thefirst and/or second means for equalizing the received signal alsorespectively have a first memory for storing a first set of filtercoefficients.

[0041] According to a further preferred development of the device, thefirst set of filter coefficients has filter coefficients which are usedin the initialization of a transmission of transmitted signals.

[0042] According to a further preferred development of the device, thefirst and/or second means for equalizing the received signal also have asecond memory for storing a second set of filter coefficients.

[0043] According to a further preferred development of the device, thesecond set of filter coefficients has filter coefficients which aresupplied by the means for setting the settable equalization.

[0044] According to a further preferred development of the device, themeans for monitoring the received signal has a filter bank, in order todetermine the energy distribution in the received signal.

[0045] According to a further preferred development of the device, thefilter bank has bandpass filters.

[0046] According to a further preferred development of the device, themeans for monitoring the received signal carries out a Fouriertransformation, in order to determine the energy distribution in thereceived signal.

[0047] According to a further preferred development of the device, thetransmitted signal has as a particular property of the particularproperties of the transmitted signal a flat baseband spectrum with alow-pass characteristic.

[0048] According to a further preferred development of the device,deviations of the transmitted signal from a flat baseband spectrum witha low-pass characteristic are taken into account in the means formonitoring the received signal in the form of a corresponding referencespectrum.

[0049] Preferred exemplary embodiments of the present invention areexplained in more detail below with reference to the accompanyingdrawings, in which:

[0050]FIG. 1 shows a general representation of the present invention;

[0051]FIG. 2 shows a first exemplary embodiment according to the presentinvention;

[0052]FIG. 3 shows a second exemplary embodiment according to thepresent invention;

[0053]FIG. 4 shows a third exemplary embodiment according to the presentinvention; and

[0054]FIG. 5 shows a fourth exemplary embodiment according to thepresent invention.

[0055] In the figures, the same reference numerals or reference numeralswhich differ only in the first digit designate component parts that arethe same or functionally the same.

[0056]FIG. 1 shows a general representation of the present invention. Atransmission system 100, in which the present invention is used,comprises a transmission channel 102, which is arranged between atransmitter 104 and a receiver, which comprises the components 112, 114and 106. When a digital transmitter 104 is used, the transmission pathof the transmission system 100 has, following the transmitter, adigital/analog converter 108, which converts the digital transmittedsignal of the transmitter 104 into an analog transmitted signal,followed by further analog components 110, by means of which the analogtransmitted signal is fed into the transmission channel 102. Thereception path also has, equivalent to this, analog reception components112, for example anti-aliasing filters, and an analog/digital converter114 for converting the analog received signal into a digital receivedsignal. The reception path may also comprise further digital components106, such as means for raising or reducing the sampling rate.

[0057]FIG. 1 also generally shows a device according to the inventionfor equalizing a received signal, which equalizes the received signal ofthe transmitter after the digital components 106. The received signal isassigned to the transmitted signal that is transmitted in the distortingtransmission system 100. The transmitted signal has time segments inwhich the same has particular properties, such as for example a whitefrequency spectrum, in the case of which successive data are notcorrelated with one another, and/or a flat frequency profile with alow-pass characteristic.

[0058] The device for equalizing the received signal has a means 116 forequalizing the received signal and for supplying an equalized receivedsignal, the means 116 for equalizing the received signal having asettable equalization. The device for equalizing the received signalalso has a means 118 for setting the settable equalization (automaticadaptation) of the means 116 for equalizing the received signal inaccordance with the received signal and a means 120 for monitoring thereceived signal and for activating the means 118 for setting thesettable equalization when the transmitted signal assigned to thereceived signal has particular properties, such as for example a whitefrequency spectrum, that are suitable for setting an equalization of thedistortion of the received signal caused by the transmission system.

[0059] The means 116 for equalizing the received signal and forsupplying a first equalized received signal preferably has aprogrammable filter 122, the equalization of which can be set by meansof filter coefficients by the means 118. The means 116 for equalizingalso has a first memory 124 for storing a first set of filtercoefficients and a second memory 126 for storing a second set of filtercoefficients. The first set of filter coefficients preferably has filtercoefficients that are used in the case of the initialization of atransmission of transmitted signals or in the case of a cold start, andmay also contain various coefficients by means of which the means 116for equalizing can be set differently during operation. The second setof filter coefficients is preferably supplied by the means 118 forsetting the settable equalization and preferably contains the momentaryequalizer setting with regard to a warm start.

[0060] For the function of the device according to the presentinvention, it is assumed that there are time segments during which thetransmitted signal has the properties required for an automaticadaptation in the means 118 for setting. By means of the means 120 formonitoring the received signal or by means of a monitor, the statisticalproperties of the transmitted signal can be monitored and the timesegments in which the transmitted signal or the assigned received signalis suitable for the adaptation can be determined. During such timesegments, an automatic coefficient adaptation can then be activated bythe means 118 for setting. In this case, different methods, known fromthe literature, can be used for the adaptation. If no suitable receivedsignal is available for the automatic adaptation, the adaptation isdeactivated by the means 118 for setting the settable equalization. Incertain time segments, the coefficients ascertained by means ofadaptation by the means 118 for setting can be taken over into theprogrammable filter 122. It is expedient to store the newly taken-overcoefficients in the second memory 126. These coefficients are thenavailable once again for example for a possible warm start. The takingover of the coefficients from the means 118 for setting into the secondmemory 126 of the means 116 for equalizing may take place at regulartime intervals, controlled by the means 118 for setting, but also by themeans 120 for monitoring the received signal by means of switches 140.It is advantageous, however, to check by means of a suitable costfunction whether the coefficient setting produces any improvement of thereception properties at all. In this way, the taking over of erroneousor unfavourable coefficient settings, which cannot be ruled out in viewof the problems described with the transmission spectrum, can beavoided.

[0061]FIG. 2 shows a first exemplary embodiment of a device forequalizing a received signal, which is assigned to a transmitted signalthat is transmitted in a distorting transmission system 200, thetransmitted signal having time segments in which the same has particularproperties, such as for example a white frequency spectrum, that aresuitable for analyzing the distortion of the received signal caused bythe transmission system 200 and for setting the equalization of thereceived signal. As already shown in FIG. 1, the transmission system 200preferably has a transmission channel 202, which is arranged between adigital transmitter 204 and a receiver, which comprises the components212, 214 and 206. Arranged downstream of the digital transmitter 204 arepreferably a digital/analog converter 208 followed by further analogcomponents 210. The reception path has analog components 212, forexample anti-aliasing filters, an analog/digital converter 214 andfurther digital components 206, such as means for raising or reducingthe sampling rate. The device for equalizing the received signalreceives the received signal from the digital components 206.

[0062] The device for equalizing a received signal has a means 216 forequalizing the received signal and for supplying a first equalizedreceived signal, which has a settable equalization. The device forequalizing a received signal also has a means 218 for setting thesettable equalization of the means 216 for equalizing the receivedsignal in accordance with the received signal and a means 220 formonitoring the received signal and for activating the means 218 forsetting when the transmitted signal assigned to the received signal hasthe particular properties.

[0063] In a way similar to in FIG. 1, the means 216 for equalizing thereceived signal preferably has a programmable filter, the equalizationof which can be set by means of filter coefficients by the means 218 forsetting. The programmable filter may in this case have a first memoryfor storing a first set of filter coefficients, which is preferably usedin the initialization (cold start) of a transmission of transmittedsignals, and a second memory for storing a second set of filtercoefficients, which is supplied by the means 218 for setting and isobtained from an adaptation or optimization.

[0064] The means 220 for monitoring the received signal preferably has afilter bank, in order to determine the energy distribution in thereceived signal. The filter bank allows evaluation of whether theestimated transmission spectrum has adequate energy in all frequencyranges, the nominal transmission spectrum serving as a reference. Inmany cases, a filter bank with a small number of bandpass filters isadequate for this. As an alternative to such a modular and regulararrangement with the known advantages with regard to implementation on asmall surface area and with little loss of performance, the means 220for monitoring the received signal may alternatively or additionallycarry out a Fourier transformation, such as for example a discreteFourier transformation or an FFT (=Fast Fourier Transform), in order todetermine the energy distribution in the received signal.

[0065] The device for equalizing a received signal also has a means 223for taking decisions and for supplying an estimate signal, whichestimates the transmitted symbols of the transmitted signal and suppliesthe estimated signals with the estimate signal, and a means 225 forsupplying an error signal ê, which determines the error signal ê fromthe difference between the estimate signal, which is supplied by themeans 223 for taking decisions, and the equalized received signal, whichis supplied by the means 216 for equalizing.

[0066] As shown in FIG. 2, the means 218 determines the settableequalization of the means 216 for equalizing the received signalautomatically from the received signal and the first error signal ê. Thesettable equalization of the means 216 for equalizing the receivedsignal is in this case preferably derived from the correlation of thereceived signal and the error signal ê. The means 220 for monitoring thereceived signal switches the error signal through to the means 218 forsetting the settable equalization for updating the setting of the means216 and activates the means 218 for setting when the transmitted signalassigned to the received signal has the particular properties, such asfor example a white frequency spectrum or adequate signal energy in allfrequency ranges. The means 220 for monitoring the received signalmonitors optionally either the equalized received signal, which issupplied by the means 216 for equalizing the received signal, or theestimate signal, which is supplied by the means 223 in the form of adecision with respect to the symbol alphabet used.

[0067]FIG. 2 presents these two possible options, which can be selectedby means of the switch 227. The first option, as mentioned, comprisesthe selection of the received signal equalized by the means 216 forequalizing before the means 223 for taking decisions, which is used forexample when the means 216 for equalizing is set close to its optimum,and the errors are in practice not correlated with the transmittedsignal, and consequently meaningful monitoring is possible. The secondoption comprises the selection of the estimate signal after the means223 for taking decisions, which is advantageous when an error-freeestimate of the transmitted symbols takes place by the means 223 fortaking decisions and therefore no error contribution enters the means220 for monitoring the received signal. In view of the in somecircumstances strong distortions in the transmission channel 202, theequalized received signal downstream of the means 223 for takingdecisions comes into consideration primarily for monitoring of thetransmission spectrum, on the basis of which the transmitted symbols canbe estimated.

[0068] The coefficient adaptation in FIG. 2 is shown for the so-calledMMSE algorithm, in the case of which the updating of the individualcoefficients is derived from the correlation of the received signal orthe input signal of the means 216 for equalizing with thecorrespondingly delayed error signal ê after the means 223 for takingdecisions. This algorithm is preferably also used in the followingexemplary embodiments.

[0069] Since it cannot be ensured how long the time segments availablefor the adaptation are and in what time interval they follow oneanother, the adaptation rate of the method is not known from the outset.Moreover, in spite of the monitoring of the data, it is not possible torule out the chance that the means 216 for equalizing has in themeantime assumed a setting that is unfavourable, for example withrespect to the mean square error (MSE) after the means 223 for takingdecisions and only very slowly converges in the direction of the optimumequalization. Losses of performance on account of such “inadvertentstates” can be obviated if an additional second means for equalizing areceived signal or an additional adaptive equalizer is provided parallelto the reception path with the first means for equalizing the receivedsignal, as described below.

[0070]FIG. 3 shows a second exemplary embodiment according to thepresent invention, and the elements shown in FIG. 3 that differ from theelements in FIG. 2 only by the first digit of the reference numeralrepresent component parts that are the same or functionally the same andare not described again below. This similarly applies to the furtherfigures that follow.

[0071] With reference to FIG. 3, the device for equalizing has inaddition to a first means 316 for equalizing the received signal and forsupplying a first equalized received signal an adaptive equalizer orsecond means 328 for equalizing the received signal and for supplying asecond equalized received signal, the second means 328 for equalizingthe received signal having a second settable equalization, which differsfrom the first settable equalization of the first means 316 forequalizing. The device for equalizing a received signal also has inaddition to the first means 323 for taking decisions and for supplying afirst estimate signal a second means 330 for taking decisions and forsupplying a second estimate signal, which estimates the transmittedsymbols of the transmitted signal and supplies the estimated symbols ina second estimate signal.

[0072] In comparison with the first exemplary embodiment of FIG. 2, thedevice for equalizing a received signal also has in addition to thefirst means 325 for supplying a first error signal ê a second means 332for supplying a second error signal ê′, which determines the seconderror signal ê′ from the difference between the first estimate signal,which is supplied by the second means 330 for taking decisions, and thesecond equalized received signal, which is supplied by the second means328 for equalizing the received signal.

[0073] In the case of this second exemplary embodiment, the means 318for setting the settable equalization of the received signal determinesthe second settable equalization of the second means 328 for equalizingthe received signal from the received signal and the second error signalê′, which is supplied by the second means 332 for supplying a seconderror signal ê′. The means 318 for setting or automatically updating thesetting of the second means 328 for equalizing preferably derives thesettable equalization from the correlation of the received signal andthe second error signal ê′.

[0074] The means 320 for monitoring the received signal switches thesecond error signal ê′ through to the means 318 for setting andactivates the means 318 for setting the settable equalization when thetransmitted signal assigned to the received signal has particularproperties, such as for example a white frequency spectrum. As in thecase of the first exemplary embodiment, the means 320 for monitoring thereceived signal optionally supplies by means of a switch 327 either thefirst equalized received signal, which is supplied by the first means316 for equalizing the received signal, or the first estimate signal,which is supplied by the first means 323 for taking decisions.

[0075] By contrast with the first exemplary embodiment, the device forequalizing a received signal according to the second exemplaryembodiment also has a means 334 for comparing, which compares a firstquality level, which is derived from the first error signal e, and asecond quality level, which is derived from the second error signal ê′,and, if the second quality level is greater than the first qualitylevel, instigates that the set equalization of the second means 328 forequalizing the received signal is supplied by the means 318 to the firstmeans 316 for equalizing the received signal by means of a switch 340.Preferably arranged upstream of the means 334 for comparing the firstand second quality levels, for ascertaining the first and second qualitylevels, are a first means 336 for ascertaining a first quality level anda second means 338 for ascertaining a second quality level, which forexample derive the first and second quality levels from the mean squareerrors of the first error signal ê and the second error signal ê′,respectively, and supply them to the means 334 for comparing.

[0076] If the second means 328 for equalizing a received signal has aprogrammable coefficient filter, the error ê′, which is ascertainedafter the second means 330 for taking decisions, which follows thesecond means 328 for equalizing a received signal, is to be used for thecoefficient adaptation of the second means 328 for equalizing thereceived signal.

[0077] As already mentioned, the transmission spectrum may deviate fromthe ideal of a flat baseband signal with a low-pass characteristic. Asalready mentioned above, this deviation is to be taken into account inthe means 220, 320 for monitoring in FIGS. 2 and 3 in the form of areference spectrum.

[0078] As in the case of the exemplary embodiments described below, animprovement of the convergence behavior is generally achieved, however,by using a shaping filter, which compensates as far as possible for thedifference from the ideal spectrum. In this way, a spectrum that is asflat as possible can be achieved for example in the transmission band. Anew reference spectrum is obtained for the signal processing downstreamof a shaping filter. There are various different ways of extending thestructures according to the present invention by adding a shapingfilter. However, it must always be ensured that only signals whichrelate to the same reference spectrum are combined, such as for examplethe correlation of a signal before a means for equalizing the errorsignal after a means for taking decisions. Therefore, it will generallybe required to insert a number of shaping filters at different points ofthe signal processing.

[0079] The introduction of a shaping filter is expedient in particularin the case of signal processing parallel to the direct reception path,such as for example of the second means for equalizing a received signalor the adaptive equalizer shown in FIG. 3, since the received spectrummust not be “shaped” in comparison with the transmission spectrum.However, insertion of a shaping filter and a downstream shaping filterwhich has a filter function that is inverse to the filter function ofthe first shaping filter into the direct reception path is not advisablein practice.

[0080]FIG. 4 shows a third exemplary embodiment of a device according tothe present invention. By contrast with the second exemplary embodiment,the means 420 for monitoring the received signal monitors optionallyeither the second equalized received signal, which is supplied by thesecond means 428 for equalizing the received signal, or the secondestimate signal, which is supplied by the second means 430 for takingdecisions, that is signals after the adaptive equalizer.

[0081] By contrast with the second exemplary embodiment of FIG. 3, thedevice for equalizing a received signal also comprises a first shapingfilter 442, which supplies a shaped received signal to the second means428 for equalizing the received signal and to the means 418 for settingor automatically updating the equalizer setting, and a second shapingfilter 444, which has a filter function corresponding to the firstshaping filter 442 and supplies a shaped second estimate signal to themeans 432 for supplying a second error signal ê″. The second shapingfilter 444, which is arranged downstream of the second means 430 fortaking decisions, ensures that the calculation of the second error ê″takes place after the second means 430 for taking decisions with respectto the same reference to which the shaped received signal for the means414 for setting the settable equalization also relates. The error ê″ascertained in this way, or the second error signal ê″ ascertained inthis way, is not identical to the ascertained error ê′ or the seconderror signal ê′ of FIG. 3. The error signal ê′ is obtained from ê″ bymeans of a shaping filter with a filter function which is inverse to thefilter function of the second shaping filter 444. The error ê″ can beused, however, in first approximation for the MMSE calculation.

[0082]FIG. 5 shows a fourth exemplary embodiment according to thepresent invention. By contrast with the third exemplary embodiment ofFIG. 4, provided in the case of this exemplary embodiment are a firstshaping filter 546, which supplies a shaped received signal to the means518 for setting the settable equalization, and a second shaping filter548, which has a filter function identical to the first shaping filter546 and supplies a shaped second error signal ê′ to the means 520 formonitoring the received signal.

[0083] Although the present invention is described above on the basis ofseveral preferred exemplary embodiments, it is not restricted to thesebut can be modified in a wide variety of ways.

[0084] List of Reference Numerals:

[0085]100 transmission system

[0086]102 transmission channel

[0087]104 digital transmitter

[0088]106 digital components

[0089]108 digital/analog converter

[0090]110 analog components

[0091]112 analog receiving components

[0092]114 analog/digital converter

[0093]116 means for equalizing the received signal

[0094]118 means for setting the settable equalization

[0095]120 means for monitoring the received signal

[0096]122 programmable filter

[0097]124 first memory

[0098]126 second memory

[0099]200 transmission system

[0100]202 transmission channel

[0101]204 digital transmitter

[0102]206 digital components

[0103]208 digital/analog converter

[0104]210 analog components

[0105]212 analog receiving components

[0106]214 analog/digital converter

[0107]216 means for equalizing the received signal

[0108]218 means for setting the settable equalization

[0109]220 means for monitoring the received signal

[0110]223 means for taking decisions

[0111]225 means for supplying

[0112]227 switches

[0113]300 transmission system

[0114]302 transmission channel

[0115]304 digital transmitter

[0116]306 digital components

[0117]308 digital/analog converter

[0118]310 analog components

[0119]312 analog receiving components

[0120]314 analog/digital converter

[0121]316 means for equalizing the received signal

[0122]318 means for setting the settable equalization

[0123]320 means for monitoring the received signal

[0124]323 first means for taking decisions

[0125]325 first means for supplying

[0126]327 switches

[0127]328 second means for equalizing the received signal

[0128]330 second means for taking decisions

[0129]332 second means for supplying

[0130]334 means for comparing

[0131]336 first means for ascertaining

[0132]338 second means for ascertaining

[0133]400 transmission system

[0134]402 transmission channel

[0135]404 digital transmitter

[0136]406 digital components

[0137]408 digital/analog converter

[0138]410 analog components

[0139]412 analog receiving components

[0140]414 analog/digital converter

[0141]416 first means for equalizing the received 418 signal

[0142]418 means for setting the settable equalization

[0143]420 means for monitoring the received signal

[0144]423 first means for taking decisions

[0145]425 first means for supplying

[0146]428 second means for equalizing the received 432 signal

[0147]430 second means for taking decisions

[0148]432 second means for supplying

[0149]434 means for comparing

[0150]436 first means for ascertaining

[0151]438 second means for ascertaining

[0152]442 first means for shaping filtering

[0153]444 second means for shaping filtering

[0154]500 transmission system

[0155]502 transmission channel

[0156]504 digital transmitter

[0157]506 digital components

[0158]508 digital/analog converter

[0159]510 analog components

[0160]512 analog receiving components

[0161]514 analog/digital converter

[0162]516 first means for equalizing the received signal

[0163]518 means for setting the settable equalization

[0164]520 means for monitoring the received signal

[0165]523 first means for taking decisions

[0166]525 first means for supplying

[0167]528 second means for equalizing the received 530 signal

[0168]530 second means for taking decisions

[0169]532 second means for supplying

[0170]534 means for comparing

[0171]536 first means for ascertaining

[0172]538 second means for ascertaining

[0173]546 first means for shaping filtering

[0174]548 second means for shaping filtering

1-30. (canceled)
 31. A device for equalizing a received signalcomprising: a first signal equalizer having a first received signalinput, a first settable coefficient used to equalize the signal, a firstcoefficient input configured to allow the first coefficient to be set,and a first equalized signal output; a coefficient setting componenthaving a first control input, and having a first coefficient outputoperably connected to the first coefficient input of the first signalequalizer, the coefficient setting component controllably operable toset the first signal equalizer coefficient; and a received signalmonitor configured to monitor the signal received by the first signalequalizer and capable of detecting a first and a second signal property,the monitor operably connected to the first control input of thecoefficient generating component, such that when the first signalproperty is detected the received signal monitor does not enable thecoefficient setting component to set the first coefficient and when thesecond signal property is detected the received signal monitor enablesthe coefficient setting component to set the first coefficient.
 32. Thedevice of claim 31, further comprising, a first signal estimator havingan input operably connected to the first equalized signal output, andhaving a first estimated signal output.
 33. The device of claim 32,further comprising, a first error detector having a first input operablyconnected to the first estimated signal output, having a second inputoperably connected to the first equalized signal output, and having afirst detected error output.
 34. The device of claim 33, wherein thecoefficient setting component is controlled by selective operableconnection of the first control input to the first detected erroroutput, the coefficient setting component further comprising: a receivedsignal input configured to receive the signal.
 35. The device of claim34, wherein the coefficient setting component correlates the firstdetected error output and the received signal to derive the firstcoefficient to be set in the first signal equalizer.
 36. The device ofclaim 34, wherein the received signal monitor is operable to selectivelyconnect the first detected error output to the first control input ofthe coefficient setting component in response to detection of the secondsignal property.
 37. The device of claim 33, further comprising, asecond signal equalizer having a second received signal input, a secondsettable coefficient used to equalize the signal, a second coefficientinput configured to allow the second coefficient to be set, and a secondequalized signal output, and wherein the coefficient setting componentfurther comprises: a second coefficient output operably connected to thesecond coefficient input of the second signal equalizer, the coefficientsetting component controllably operable to set the second signalequalizer coefficient.
 38. The device of claim 37, further comprising, asecond signal estimator having an input operably connected to the secondequalized signal output, and having a second estimated signal output.39. The device of claim 38, further comprising, a second error detectorhaving a first input operably connected to the second estimated signaloutput, having a second input operably connected to the second equalizedsignal output, and having a second detected error output.
 40. The deviceof claim 39, the coefficient setting component further comprising: asecond control input, the second control input capable of selectiveconnectivity to the output of the second error detector.
 41. The deviceof claim 40, wherein the coefficient generating component correlates thereceived signal and the output of the second error detector to derivethe second coefficient.
 42. The device of claim 41, wherein the receivedsignal monitor is operable to selectively connect the second detectederror output to the second control input of the coefficient settingcomponent in response to detection of the second signal property. 43.The device of claim 42, further comprising: a switch operably connectedto the first coefficient input of the first signal equalizer and havinga first switch position operably connected to the first coefficientoutput of the coefficient setting component and a second switch positionoperably connected to the second coefficient output of the coefficientsetting component; and a comparator having a first input operablyconnected to the first detected error output, having a second inputoperably connected to the second detected error output, and having acomparison condition output controllably connected to the switch, thecomparator operable to generate a first and a second comparisoncondition based upon the first and the second detected error output,such that when the first comparison condition is generated the switch isplaced in the first switch position and when the second comparisoncondition is generated the switch is placed in the second switchposition.
 44. The device of claim 43, further comprising: a first meansfor ascertaining a quality level having an input operably connected tothe first detected error output, and having a quality level outputoperably connected to the first input of the comparator; and a secondmeans for ascertaining a quality level having an input operablyconnected to the second detected error output, and having a qualitylevel output operably connected to the second input of the comparator.45. The device of claim 44, wherein the quality level of the first meansfor ascertaining a quality level comprises a mean square error of thefirst detected error output and the quality level of the second meansfor ascertaining a quality level comprises a mean square error of thesecond detected error output.
 46. The device of claim 39, furthercomprising: a first shaping filter having an input operably connected toreceive the signal and an output operably connected to the secondequalizer and the coefficient setting component, such that the signalreceived by the second equalizer and the coefficient setting componentis a filtered received signal; and a second shaping filter having aninput operably connected to the output of the second signal estimator,and an output operably connected to the input of the second errordetector, and having a filter function corresponding to the filterfunction of the first shaping filter.
 47. The device of claim 39,further comprising: a first shaping filter having an input operablyconnected to receive the signal and a filtered output operably connectedto the received signal input of the coefficient setting component, suchthat the signal received by the coefficient setting component is afiltered received signal; and a second shaping filter having an inputoperably connected to the second detected error output, and a filteredoutput selectively connectable to the second control input of thecoefficient setting component, and having a filter functioncorresponding to the filter function of the first shaping filter. 48.The device of claim 38, further comprising; a switch operably connectedto the received signal monitor and having a first position operablyconnected to the second equalized signal output, and having a secondposition operably connected to the second estimated signal output, suchthat when the switch is in the first position, the received signalmonitor monitors the second equalized received signal and when theswitch is in the second position the received signal monitor monitorsthe second estimated signal output.
 49. The device of claim 37, thefirst signal equalizer further comprising, a filter, and wherein thefirst settable coefficient comprises a programmable input of the filter.50. The device of claim 49, further comprising: a first memory area forstoring a first at least one filter coefficient.
 51. The device of claim50, wherein the first at least one filter coefficient comprises a filtercoefficient selected for use in the initialization of the transmissionof a signal.
 52. The device of claim 51, further comprising; a secondmemory area for storing a second at least one filter coefficient. 53.The device of claim 52, wherein the second at least one filtercoefficient is passed to the second memory area from the coefficientsetting component.
 54. The device of claim 32, further comprising; aswitch operably connected to the received signal monitor and having afirst position operably connected to the first equalized signal output,and having a second position operably connected to the first estimatedsignal output, such that when the switch is in the first position, thereceived signal monitor monitors the first equalized signal output andwhen the switch is in the second position the received signal monitormonitors the first estimated signal output.
 55. The device of claim 31,the received signal monitor further comprising: a filter bank, thefilter bank operable to detect energy distribution in the receivedsignal.
 56. The device of claim 55, wherein the filter bank comprisesbandpass filters.
 57. The device of claim 31, wherein the receivedsignal monitor performs a Fourier transformation to detect energydistribution in the received signal.
 58. The device of claim 31, thereceived signal monitor further comprising: a reference spectrum, thereference spectrum having the form of a flat baseband signal with alow-pass characteristic, and wherein monitoring the signal received bythe device comprises a comparison of the received energy to thereference spectrum
 59. The device of claim 31, wherein the input signalmonitor performs a statistical analysis of at least one detected signalproperty.
 60. A transmission system comprising: a transmitter; areceiver; a transmission channel arranged between the transmitter andthe receiver; and an equalizer, the equalizer comprising: a means forequalizing a received signal, the means for equalizing comprising atleast one programmable coefficient; a means for programming the at leastone coefficient in the means for equalizing, the means for programminghaving an activated state wherein the at least one coefficient isprovided to the means for equalizing and an non-activated state whereinthe at least one coefficient is not provided to the means forequalizing; and a means for monitoring the received signal, the meansfor monitoring operable to perform statistical analysis of the receivedsignal so as to determine if a first signal characteristic is present inthe received signal and, when the first signal characteristic ispresent, to activate the means for programming and, when the firstsignal characteristic is not present, to de-activate the means forprogramming.
 61. A method of setting equalization for a received signalcomprising the steps of: equalizing a received signal; monitoring thereceived signal; detecting a property of the signal; enablingmodification of the equalization of the received signal during the timethat the property is detected; analyzing the equalized received signaland modifying the equalization of the received signal based upon theanalysis.
 62. The method of claim 61, further comprising, after the stepof detecting, the steps of: determining that the property is notpresent; and disabling the modification of the equalization of thereceived signal.
 63. The method of claim 62, wherein the step ofdetecting comprises the step of: detecting that the received signal hasa white frequency spectrum, and wherein the step of enablingmodification comprises the step of: enabling modification of theequalization of the received signal during the time that the whitefrequency spectrum is detected.
 64. The method of claim 63, wherein thestep of detecting that the received signal has a white frequencyspectrum comprises the step of: performing a Fourier transformation ofthe received signal.